1. Technical Field
The present invention relates to a projection zoom lens system of a projector apparatus for projecting a magnified image displayed on a light valve onto a screen.
2. Description of the Related Art
FIG. 7 shows a schematic structure of a typical three-plate type liquid crystal (LCD) projector. A projector apparatus 8 for projecting a magnified image onto a screen or the like includes a projection zoom lens system 1 for projecting an image supplied from incident side onto the screen, and an image forming apparatus 7 for supplying the image to the projection zoom lens system 1. In the LCD projector apparatus, LCD 3 is employed as a light valve of the image forming apparatus 7. The projector apparatus 8 shown in FIG. 7 includes a white light source 6, dichroic mirrors 5R and 5G for color-separating the light from the light source 6, liquid crystal panels 3R, 3G and 3B. LCD's are light through type display medium (light valve) and an image of each color, namely red, green and blue are generated respectively. Images formed by these LCD's 3R, 3G and 3B are introduced into a dichroic prism 2 by a mirror 4, color-compounded and led to the incidence side of the projection lens system 1. Then, images displayed by the LCD light valves 3B, 3G and 3R are magnified and compounded and projected onto a screen 9.
The projection zoom lens system used in such a LCD projector must have a long back focus for inserting the dichroic prism 2. That is, since the characteristics of dichroic prism largely depends on an incident angle because of spectral characteristics, the liquid crystal panel side, namely the incidence side of the projection zoom lens system 1 has to be telecentric. Further, not only in the above three-plate type image-forming apparatus, but also in any image-forming apparatus using LCD light valve, it is preferable to apply the projection lens system having the telecentric incidence side. Because the visual angle of the liquid crystal panel is relatively small, the image quality depends highly on the angle.
In recent years, in place of the liquid crystal panels, the apparatus having a plurality of elements for forming an image by mechanically changing reflecting direction of light using microelectromechanical systems have been brought into practical use. One of examples of such apparatus is a DMD (digital mirror device, deformation micro-foil device or display) in which fine mirrors (micro mirrors) are arranged into an array in correspondence with pixels for displaying image by controlling angle of each of the mirrors. An image-forming apparatus for forming pixels by the micro mirrors provides faster response speed and brighter and high-contrast image than those of LCD and thus, is suitable for providing a compact projector of high-intensity and high-quality image.
As a projector apparatus employing the aforementioned DMD as a light valve, there has also been developed a single-plate type projector apparatus using a rotation color filter and a TIR prism in an optical system for illuminating on DMD on and output the image from the DMD. In this DMD projector apparatus, the characteristic of the prism greatly depends on the incident angle has not been changed. Therefore, not only the LCD projector apparatus but also the DMD projector apparatus require a projection zoom lens system having a telecentric incident side.
In recent years, a projector apparatus being compact, light-weighted, thin and easy to carry has been increasingly required. Accompanied with this trend, the smaller size and higher-performance projection zoom lens system has been further required. That is, the small and wide-angle projection zoom lens system suitable for high-intensity and high-resolution image, and having sufficient relative illumination is needed.
Thereupon, it is an object of the present invention to provide a projection lens system that is small, especially small in diameter, bright as the F-number is about 2.0 at the wide-angle end, and wide angle as the half field angle is 25 degree or larger. In addition to the above feature above, it is an object of this invention to provide the projection lens system having sufficient relative illumination and excellent aberration performance. Further, it is another object of the invention to provide a compact projector apparatus capable of displaying a bright, high resolution and high contrast image using the thin and small projection zoom lens system of this invention.